Solar cells are generally made of semiconductor materials, which convert sunlight into electricity. A conventional silicon solar cell is typically made of a thin wafer of generally p-type base silicon (Si) having a negative electrode on the front side or sun-facing side and a positive electrode on the back side. Alternatively, n-type Si wafers can be used with adjustment in the front-side doping to p-type, for example, using a boron emitter. A p-n junction can be fabricated by diffusing phosphorus (P) from a suitable phosphorus source into the p-type Si wafer. An anti-reflective coating (ARC) is generally applied on the top of the front side of the solar cell to prevent reflective loss of sunlight. An ARC can be a silicon nitride layer deposited by plasma-enhanced chemical vapor deposition. Radiation of an appropriate wavelength impinging on the semiconductor serves as a source of external energy to generate electron-hole pairs in the base of the solar cell. Due to the potential difference at the p-n junction, holes and electrons move across the junction in opposite directions to generate an electric current. The current is collected by a conductive grid/metal contact applied to the surfaces of the silicon semiconductor and directed to external circuitry.
In general, for Si solar cell metallization applications, thick-film pastes are used to form the conductive grids or metal contacts. Thick-film pastes can include a suspension of conductive metal, glass frit, organic vehicles, and modifiers. Silver is the most common conductive filler used for front-side contact paste. Glass frit are used to bind the functional/conductive phase to a silicon wafer after thermal treatment. The glass frit also etches through anti-reflective and passivation layers to provide ohmic contact between the silver grid and the silicon surface of the solar cell. The vehicle is an organic system that acts as a rheological carrier to control flow and printability of the paste. The organic vehicle is composed of resins, solvents, and additives. The attributes of the metallization paste, in particular the front-side silver paste, is important for achieving high efficiency solar cells. Therefore, the quality and performance of the silver metallization paste affects the economics of solar systems. The screen printing technology that is currently used for the majority of solar cells is being further developed for printing fine-line conductive grids to reduce silver consumption and thereby reduce cost, to reduce shading and thereby increase the current density, and to improve photovoltaic module performance.
To maintain high efficiencies over time it is important that the pull strength and the adhesion of the solar cell metallization be robust. Achieving low specific contact resistance on lightly-doped high sheet resistance Si emitters (about 100 Ω/sq or greater) for thick-film screen-printed metallization depends on a number of interrelated factors that are not fully understood. To also achieve high pull strength is challenging because as the weight percent of the glass frit, which acts as the high temperature binder, is increased to improve the adhesion strength, the contact resistance of the conductor also increases resulting in the concomitant decrease in the fill factor (FF) and the conversion efficiency for the solar cell. Also, increasing the amount of one of the main glass network formers such as the lead oxide will decrease the quality of the p-n junction, which reduces the conversion efficiency. Increasing the quantity of printed metallization paste to increase the final pull strength of the fired conductor is also not desirable because of increased material costs.
Silver metallization pastes containing glass frit formed using various metal oxides such as disclosed in U.S. Application Publication No. 2015/0364622 have not demonstrated improved electrical performance compared to conductors prepared from pastes without the metal oxide frit.
To provide an economical process for manufacturing high efficiency photovoltaic cells, there is a need for a thick-film metallization paste that can be screen-printed to provide conductive grids with small feature dimensions and a high aspect ratio with printability resolution, and that exhibit low resistivity, high adhesion strength to the semiconductor substrate as well as between the Ag bulk and the glass at the contact interface, and excellent solderability. The fired screen-printed metallization should also exhibit a low specific contact resistance, excellent junction ideality, and high fill factor (FF).